1. Field of the Invention
The present invention relates to a novel heat-sensitive recording material. More particularly, it relates to a heat-sensitive recording material showing reduced fog and improved image preservation.
2. Description of the Related Art
Recently, heat-sensitive recording materials comprising a heat-sensitive color forming layer coated on a supporter, such as paper, synthetic paper, plastic film or the like, have widely been used in thermal printers of facsimiles, electronic calculators, microcomputers and the like, in thermo-pen recorders of cardiograph, analytical instruments and the like, as train tickets, POS labels in super markets and the like, and in the like other applications.
These heat-sensitive recording materials are generally produced by first preparing coating fluid, then coating the fluid on a supporter such as paper and drying to form a heat-sensitive color forming layer. For the preparation of the coating fluid, a colorless or light color leuco dyestuff of a lactone, a lactam, a spiropyran or the like as a color forming substance and a developer which develops color of the color forming substance by reaction with it when heated are separately pulverized with a ball mill or a sand mill and dispersed in a medium. The dispersions are mixed together with addition of a binder to prepare a coating fluid. Wax, sensitizers, surface active agents, defoaming agents, inorganic pigments and the like may be added to the mixed dispersion according to necessity.
As a developer showing good image preservation, 2,4'-dihydroxydiphenylsulfone or 4,4'-dihydroxydiphenylsulfone is used in Japanese Patent Application Laid-Open Nos. 1982-150599 and 1988-3991. However, these compounds show inferior heat response and sufficient color density cannot be obtained. To solve this problem, a sensitizer is generally added to the developer to lower the color forming temperature. Examples of the sensitizer currently used for such purpose are paraffin wax, amides, such as fatty acid amides, esters, such as dimethyl terephthalate, and ethers. However, these sensitizers often have drawbacks that they do not always provide sufficiently high color forming density, color forming sensitivity and reduced fog and that image preservation is poor to result in deterioration of the image with time even when an image of high density can be obtained. Thus, no sensitizer having satisfactory properties has heretofore been obtained.